Fix initial window size on Wayland

[jeremypw]

Fixes #762

• Applies the hard-coded minimum window dimensions as a fallback when window dimensions are restored.
• Fixes size of second and subsequent window by connecting to configure event after restoring state and only for first window. Removes separate code for sizing new windows.
• Avoids a theoretically possible dereference of 'null' on new window action.

Should be tested on OS8/Wayland, OS8/X as well as OS7.1

[leolost2605]

I think a better fix would be to use the get_monitor_at_window instead of the primary one which is

• supported on Wayland
• usually the primary one anyways on first launch
• and if not it makes more sense to adjust the size according to the monitor it's actually on and not to the primary one

(Though I have to say I don't know anything about the terminal codebase 😅)

[jeremypw]

@leolost2605 I'll try it. I was assuming it was the get_geometry () that was the problem tho` - might be as well to have an ultimate hard-coded fallback still.

[jeremypw]

The problem with using get_monitor_at_window () and get_primary_monitor () is that it does not work unless the window is realized (which it isn't at the moment). However if you restore the window size after realizing then setting the default dimensions does not work. In Gtk3 you can use resize () but that does not exist in Gtk4 so I do not want to introduce it here. I think therefore we will have to abandon the idea of resizing the terminal window according to the monitor size as a fallback

Substantial changes made since review